diff options
| -rw-r--r-- | include/linux/wait.h | 30 |
1 files changed, 30 insertions, 0 deletions
diff --git a/include/linux/wait.h b/include/linux/wait.h index 1e1bf9f963a9..f3bac30587f7 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -102,6 +102,36 @@ init_waitqueue_func_entry(wait_queue_t *q, wait_queue_func_t func) q->func = func; } +/** + * waitqueue_active -- locklessly test for waiters on the queue + * @q: the waitqueue to test for waiters + * + * returns true if the wait list is not empty + * + * NOTE: this function is lockless and requires care, incorrect usage _will_ + * lead to sporadic and non-obvious failure. + * + * Use either while holding wait_queue_head_t::lock or when used for wakeups + * with an extra smp_mb() like: + * + * CPU0 - waker CPU1 - waiter + * + * for (;;) { + * @cond = true; prepare_to_wait(&wq, &wait, state); + * smp_mb(); // smp_mb() from set_current_state() + * if (waitqueue_active(wq)) if (@cond) + * wake_up(wq); break; + * schedule(); + * } + * finish_wait(&wq, &wait); + * + * Because without the explicit smp_mb() it's possible for the + * waitqueue_active() load to get hoisted over the @cond store such that we'll + * observe an empty wait list while the waiter might not observe @cond. + * + * Also note that this 'optimization' trades a spin_lock() for an smp_mb(), + * which (when the lock is uncontended) are of roughly equal cost. + */ static inline int waitqueue_active(wait_queue_head_t *q) { return !list_empty(&q->task_list); |